1. Field of the Invention
The present invention relates to a foamable silicone rubber composition that can be foamed and cured by ultra high Frequency vulcanization (hereinafter referred to as UHF vulcanization).
2. Description of the Prior Art
Recently, in the field of producing thick-walled or block foams such as foamed building gaskets, foamed insulating materials and foamed rolls, it is strongly required that these foam products are produced economically and stably by continuous extrusion. In the case of ethylene/propylene/diene rubber (EPDM) and chloroprene rubber (CR), for instance, as a production method capable of meeting the above requirement, a method in which a continuously extruded product is subjected to UHF vulcanization, has been put to practical use.
The UHF vulcanization is conducted by exposing a material to be treated to a microwave of 2,450.+-.50 MHz or 915.+-.25 MHz in a microwave heating oven. The material absorbs the microwave and generates heat, thus being foamed and cured. The value of generated heat (p) (watt/m.sup.2) is expressed by the following equation: ##EQU1## wherein f represents the frequency of a microwave, E represents the high-frequency electric field (V/m), .epsilon. represents the relative dielectric constant of the material to be treated, and tan.delta. represents dielectric loss tangent. It has been so far said that the UHF vulcanization is favorably applicable to materials having a dielectric loss index (.epsilon..multidot.tan.EPSILON.) of 0.08 or more.
Recently, also in regard to silicones, in order to produce thick-walled or block silicone rubber foams stably and economically by continuous extrusion, conventional production processes are reconsidered so that a new foaming and curing technique may be developed.
Heretofore, as a method of continuous vulcanization, the hot-air vulcanization (HAV) method under normal pressure, the continuous steam vulcanization (CV) method and the liquid curing medium (LCM) method are known. However, these methods have the disadvantages that they are not suited to produce foams of continuous length, and the state of cells in the foams is not good.
It is also said that to apply UHF vulcanization to silicone rubbers, is substantially impossible because the silicone rubbers generally have a loss index of about 0.03 at 3 GHz.
The Japanese Pre-examination Patent Publication (KOKAI) No. 37963/1977 discloses a method in which an organopolysiloxane composition is cured with a microwave. In the organopolysiloxane as the base component of the organopolysiloxane composition used in this method, the allyl group, fluorinated aliphatic hydrocarbon groups, mercapto group-containing hydrocarbon groups, carbinolgroup containing hydrocarbon groups or aliphatic hydrocarbon ether groups account for 5 mol% or more of all the organic groups bonded to silicon atoms. The organopolysiloxane composition has the disadvantage that heat resistance, weather resistance and electric properties are deteriorated in the cured product of the composition, although these properties are inherently possessed by silicone compositions. Therefore, the composition is not suitable in practical use.